This invention relates to the manufacture of electron tubes and particularly to an improved method for air letting an evacuated cathode ray tube.
Cathode ray tubes basically comprise a cathodoluminescent screen and at least one electron beam generating means contained within an evacuated envelope. The conventional cathode ray tube comprises a glass envelope having a funnel-shaped portion, the expansive section thereof having a terminal closure in the form of a faceplate panel with the cathodoluminescent screen disposed relative to the interior surface thereof, and the other end narrowing down into a neck portion. Within the neck portion, one or more electron guns are positioned in a manner to beam electrons to the screen. The neck portion normally terminates in a stem having an array of metallic pins extending therefrom which effect external electrical connections for the several internal components of the tube.
There are occasions when a tube is found to contain a malfunctioning or defective component, such as an electron gun assembly. A situation of this type may arise during tube manufacturing or as a result of tube operation. When the other components of the tube are considered to be satisfactory, it is the conventional and economical practice to repair the tube by removing the defective gun assembly and replacing it with a new one. Basically, this process involves disturbing the vacuum of the tube, removing defective electron gun assembly by cutting off a portion of the neck of the tube, resealing a new neck portion thereto, sealing a new mount assembly into the re-necked tube and exhausting and processing the repaired tube in accordance with conventional processing and aging practices.
The initial step in the repairing operation, as noted above, is that of disturbing the vacuum wherein the evacuated tube is opened and a fill of an inert, noncontaminating gas is admitted to prevent an inflow of contaminating gases such as oxygen, carbon dioxide, water vapor and the like, which, if sorbed by the internal parts of the tube, would be detrimental to subsequent tube operation. It has been conventional practice to carefully puncture the neck of the tube in the presence of a surrounding noncontaminating atmosphere and allow this atmosphere to flow through the opening and fill the tube. Usually, the puncture operation is consumated in one of two ways, either by utilizing a heated metallic tip to melt a hole in the glass or by boring a hole therethrough by means of a rapidly revolving drill. Both of these methods have disadvantages in that heat releases gaseous contaminants such as water vapor and objectionable hydroxides from the adjacent glass, and drilling produces fine chips and particle glass materials. Thus, as the vacuum in the tube is disturbed, the aforementioned and other deleterious gas borne foreign materials, ambient to the tube, are undesirably sucked into the interior of the envelope, and are undesirably sorbed or adhered to the surfaces included therein. While a portion of these contaminants can be removed by subsequent tube processing, for example by knocking, some of the contaminants remain as potentially troublesome deterrents to desirable performance of the repaired tube.
Another method sought to circumvent the aforementioned problems by breaking the glass exhaust tubulation or evacuation seal within an evacuated hermetic chamber, as disclosed in U.S. Pat. No. 3,404,933 issued to R. J. Weidman. Although this method solved the problem of the initial surge of incoming gas sweeing contaminating particles and other contaminants into the tube, the breaking of the exhaust tubulation by impact from a plunger generates many small glass particles, some of which may enter the tube as a result of the impact force of the plunger and be further swept into the tube by the controlled emission of the inert gas. In addition, this method adds at least one superfluous step, that of smashing the exhaust tubulation, since the neck of the tube must ultimately be cracked and the mount assembly removed before tht tube can be re-necked and a new mount assembly installed.